1. Field of the Invention
The present invention relates to systems and methods for removing materials from flue gas, and, more particularly, to improved systems and methods for flue gas denitrification (i.e., for removing nitrogen oxides from flue gas) via regenerative selective catalytic reduction (RSCR).
2. Description of Related Art
High-temperature combustion processes and other like technologies serve vital roles in industry; however, often an unfortunate by-product of such processes is the generation and release into the atmosphere of contaminants within outputted flue gas. Among the most notorious of these contaminants are nitrogen oxides (hereinafter referred to as “NOx”), which are classified as pollutants by the EPA, and the output of which has been linked to the generation of smog and so-called acid rain. Thus, it is a common goal of those in industry to reduce to acceptable levels the amount of contaminants such as NOx within outputted flue gas.
For years, a commonly employed technique for reducing NOx emissions was to modify the combustion process itself, e.g., by flue gas recirculation. However, in view of the generally poor proven results of such techniques (i.e., NOx removal efficiencies of 50% or less), recent attention has focused instead upon various flue gas denitrification processes (i.e., processes for removing nitrogen from flue gas prior to the flue gas being released into the atmosphere).
Flue gas denitrification processes are categorized into so-called “wet” methods, which utilize absorption techniques, and “dry” methods, which instead rely upon adsorption techniques, catalytic decomposition and/or catalytic reduction. At present, a widely implemented denitrification process is selective catalytic reduction (SCR), which is a “dry” denitrification method whereby the introduction of a reactant (e.g., NH3) causes reduction of the NOx, which, in turn, becomes transformed into harmless reaction products, e.g., Nitrogen and water. The reduction process in an SCR process is typified by the following chemical reactions:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2O6NO2+8NH3→7N2+12H2ONO+NO2+2NH3→2N2+3H2O
Due to the technology involved in SCR, there is some flexibility in deciding where to physically site the equipment for carrying out the SCR process. In other words, the chemical reactions of the SCR process need not occur at a particular stage or locus within the overall combustion system. The two most common placement sites are within the midst of the overall system (i.e., on the “hot side”), or at the so-called “tail end” of the overall system (i.e., on the “cold side”).
Unfortunately, significant problems are encountered in industrial settings with respect to both hot side and cold side SCR installations. For example, hot side SCR processes are not optimal for use in conjunction with wood-fired burners. This is because ash present within the wood contains alkalis, which can cause damage to the catalyst due to deactivating the catalyst surface, either by blinding or chemical reaction. Cold side SCR processes avoid this disadvantage because they are located downstream of the particulate control device, but are plagued by thermal inefficiency due to their reliance on indirect heat exchangers.
Thus, a need exists for a selective catalytic reduction process that can be easily implemented into existing industrial operations, and that allows effective and efficient removal of NOx from flue gas while achieving high thermal efficiency and minimizing significant installation- and/or operation-related costs.